Cash shells tend to be companies that have fallen on hard times; their assets are sold off (liabilities settled) and the company is run as a going concern. Due to the fact that the company has a stockmarket listing and cash on the balance sheet, it is attractive to those seeking a listing ofr their businesses and a slightly wider shareholder base.
They can be very rewarding for new shareholders that buy in at the right price before they rocket. However, these are relatively small companies that do not have anything going for them and as such are high risk. They are strictly for gamblers. Bid/offer spreads are generally very wide and it is almost impossible to deal in significant volumes. They tend to be penny shares.
That said, they are listed on Aim; FTSE Fledglings and FTSE SmallCap. The Penny Share Guide tends to give regular tips and analysis to subscribers and to those looking for a punt they could do worse than take out a subscription.